BitOp

# API Functions

This list of API functions is not intended to replace a tutorial. If you are not familiar with the terms used, you may want to study the » Wikipedia article on bitwise operations first.

## Loading the BitOp Module

The suggested way to use the BitOp module is to add the following to the start of every Lua file that needs one of its functions:

    local bit = require("bit")

This makes the dependency explicit, limits the scope to the current file and provides faster access to the `bit.*` functions, too. It's good programming practice not to rely on the global variable `bit` being set (assuming some other part of your application has already loaded the module). The `require` function ensures the module is only loaded once, in any case.

## Defining Shortcuts

It's a common (but not a required) practice to cache often used module functions in locals. This serves as a shortcut to save some typing and also speeds up resolving them (only relevant if called hundreds of thousands of times).

    local bnot = bit.bnot
    local band, bor, bxor = bit.band, bit.bor, bit.bxor
    local lshift, rshift, rol = bit.lshift, bit.rshift, bit.rol
    -- etc...

    -- Example use of the shortcuts:
    local function tr_i(a, b, c, d, x, s)
      return rol(bxor(c, bor(b, bnot(d))) + a + x, s) + b
    end

Remember that `and`, `or` and `not` are reserved keywords in Lua. They cannot be used for variable names or literal field names. That's why the corresponding bitwise functions have been named `band`, `bor`, and `bnot` (and `bxor` for consistency).

While we are at it: a common pitfall is to use `bit` as the name of a local temporary variable — well, don't! :-)

## About the Examples

The examples below show small Lua one-liners. Their expected output is shown after `-->`. This is interpreted as a comment marker by Lua so you can cut & paste the whole line to a Lua prompt and experiment with it.

Note that all bit operations return signed 32 bit numbers (rationale). And these print as signed decimal numbers by default.

For clarity the examples assume the definition of a helper function `printx()`. This prints its argument as an unsigned 32 bit hexadecimal number on all platforms:

    function printx(x)
      print("0x"..bit.tohex(x))
    end

## Bit Operations

### `y = bit.tobit(x)`

Normalizes a number to the numeric range for bit operations and returns it. This function is usually not needed since all bit operations already normalize all of their input arguments. Check the operational semantics for details.

    print(0xffffffff)                --> 4294967295 (*)
    print(bit.tobit(0xffffffff))     --> -1
    printx(bit.tobit(0xffffffff))    --> 0xffffffff
    print(bit.tobit(0xffffffff + 1)) --> 0
    print(bit.tobit(2^40 + 1234))    --> 1234

(*) See the treatment of hex literals for an explanation why the printed numbers in the first two lines differ (if your Lua installation uses a `double` number type).

### `y = bit.tohex(x [,n])`

Converts its first argument to a hex string. The number of hex digits is given by the absolute value of the optional second argument. Positive numbers between 1 and 8 generate lowercase hex digits. Negative numbers generate uppercase hex digits. Only the least-significant 4*|n| bits are used. The default is to generate 8 lowercase hex digits.

    print(bit.tohex(1))              --> 00000001
    print(bit.tohex(-1))             --> ffffffff
    print(bit.tohex(0xffffffff))     --> ffffffff
    print(bit.tohex(-1, -8))         --> FFFFFFFF
    print(bit.tohex(0x21, 4))        --> 0021
    print(bit.tohex(0x87654321, 4))  --> 4321

### `y = bit.bnot(x)`

Returns the bitwise not of its argument.

    print(bit.bnot(0))            --> -1
    printx(bit.bnot(0))           --> 0xffffffff
    print(bit.bnot(-1))           --> 0
    print(bit.bnot(0xffffffff))   --> 0
    printx(bit.bnot(0x12345678))  --> 0xedcba987

### `y = bit.bor(x1 [,x2...])
y = bit.band(x1 [,x2...])
y = bit.bxor(x1 [,x2...])`

Returns either the bitwise or, bitwise and, or bitwise xor of all of its arguments. Note that more than two arguments are allowed.

    print(bit.bor(1, 2, 4, 8))                --> 15
    printx(bit.band(0x12345678, 0xff))        --> 0x00000078
    printx(bit.bxor(0xa5a5f0f0, 0xaa55ff00))  --> 0x0ff00ff0

### `y = bit.lshift(x, n)
y = bit.rshift(x, n)
y = bit.arshift(x, n)`

Returns either the bitwise logical left-shift, bitwise logical right-shift, or bitwise arithmetic right-shift of its first argument by the number of bits given by the second argument.

Logical shifts treat the first argument as an unsigned number and shift in 0-bits. Arithmetic right-shift treats the most-significant bit as a sign bit and replicates it.
Only the lower 5 bits of the shift count are used (reduces to the range [0..31]).

    print(bit.lshift(1, 0))              --> 1
    print(bit.lshift(1, 8))              --> 256
    print(bit.lshift(1, 40))             --> 256
    print(bit.rshift(256, 8))            --> 1
    print(bit.rshift(-256, 8))           --> 16777215
    print(bit.arshift(256, 8))           --> 1
    print(bit.arshift(-256, 8))          --> -1
    printx(bit.lshift(0x87654321, 12))   --> 0x54321000
    printx(bit.rshift(0x87654321, 12))   --> 0x00087654
    printx(bit.arshift(0x87654321, 12))  --> 0xfff87654

### `y = bit.rol(x, n)
y = bit.ror(x, n)`

Returns either the bitwise left rotation, or bitwise right rotation of its first argument by the number of bits given by the second argument. Bits shifted out on one side are shifted back in on the other side.
Only the lower 5 bits of the rotate count are used (reduces to the range [0..31]).

    printx(bit.rol(0x12345678, 12))   --> 0x45678123
    printx(bit.ror(0x12345678, 12))   --> 0x67812345

### `y = bit.bswap(x)`

Swaps the bytes of its argument and returns it. This can be used to convert little-endian 32 bit numbers to big-endian 32 bit numbers or vice versa.

    printx(bit.bswap(0x12345678)) --> 0x78563412
    printx(bit.bswap(0x78563412)) --> 0x12345678

## Example Program

This is an implementation of the (naïve) Sieve of Eratosthenes algorithm. It counts the number of primes up to some maximum number.

A Lua table is used to hold a bit-vector. Every array index has 32 bits of the vector. Bitwise operations are used to access and modify them. Note that the shift counts don't need to be masked since this is already done by the BitOp shift and rotate functions.

    local bit = require("bit")
    local band, bxor = bit.band, bit.bxor
    local rshift, rol = bit.rshift, bit.rol

    local m = tonumber(arg and arg[1]) or 100000
    if m < 2 then m = 2 end
    local count = 0
    local p = {}

    for i=0,(m+31)/32 do p[i] = -1 end

    for i=2,m do
      if band(rshift(p[rshift(i, 5)], i), 1) ~= 0 then
        count = count + 1
        for j=i+i,m,i do
          local jx = rshift(j, 5)
          p[jx] = band(p[jx], rol(-2, j))
        end
      end
    end

    io.write(string.format("Found %d primes up to %d\n", count, m))

Lua BitOp is quite fast. This program runs in less than 90 milliseconds on a 3 GHz CPU with a standard Lua installation, but performs more than a million calls to bitwise functions. If you're looking for even more speed, check out » LuaJIT.

## Caveats

### Signed Results

Returning signed numbers from bitwise operations may be surprising to programmers coming from other programming languages which have both signed and unsigned types. But as long as you treat the results of bitwise operations uniformly everywhere, this shouldn't cause any problems.

Preferably format results with `bit.tohex` if you want a reliable unsigned string representation. Avoid the `"%x"` or `"%u"` formats for `string.format`. They fail on some architectures for negative numbers and can return more than 8 hex digits on others.

You may also want to avoid the default number to string coercion, since this is a signed conversion. The coercion is used for string concatenation and all standard library functions which accept string arguments (such as `print()` or `io.write()`).

### Conditionals

If you're transcribing some code from C/C++, watch out for bit operations in conditionals. In C/C++ any non-zero value is implicitly considered as "true". E.g. this C code:
`if (x & 3) ...`
must not be turned into this Lua code:
`if band(x, 3) then ... -- wrong!`

In Lua all objects except `nil` and `false` are considered "true". This includes all numbers. An explicit comparison against zero is required in this case:
`if band(x, 3) ~= 0 then ... -- correct!`

### Comparing Against Hex Literals

Comparing the results of bitwise operations (signed numbers) against hex literals (unsigned numbers) needs some additional care. The following conditional expression may or may not work right, depending on the platform you run it on:
`bit.bor(x, 1) == 0xffffffff`
E.g. it's never true on a Lua installation with the default number type. Some simple solutions:

- Either never use hex literals larger than 0x7fffffff in comparisons:
  `bit.bor(x, 1) == -1`
- Or convert them with `bit.tobit()` before comparing:
  `bit.bor(x, 1) == bit.tobit(0xffffffff)`
- Or use a generic workaround with `bit.bxor()`:
  `bit.bxor(bit.bor(x, 1), 0xffffffff) == 0`
- Or use a case-specific workaround:
  `bit.rshift(x, 1) == 0x7fffffff`

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